A.
All land development within the Township shall be designed, implemented,
operated and maintained to meet the purposes of this chapter, through
these two elements:
B.
No approval of any subdivision or land development plans that contain regulated activities, or issuance of any building or occupancy permit, or the commencement of any earth disturbance 5,000 square feet or greater at a project site within the Township, shall proceed until the requirements of this chapter are met, including approval of an individual stormwater management project plan under § 280-10 and a permit under PADEP regulations, where applicable.
(1)
Exemptions for small developments.
(a)
At the time of application, the Township shall determine if
the subdivision/land development qualifies as a small development
and, therefore, is eligible for a simplified stormwater management
project plan submission.
(b)
For the purpose of this chapter, a "small development" is any
subdivision or land development which results (or will result when
fully developed) in the creation of 5,000 square feet or less of impervious
area and one acre or less of any land cover changes.
A.
All regulated earth disturbance activities shall be subject to standards
and possible permit requirements by PADEP under regulations at 25
Pa. Code Chapter 102. All projects that disturb greater than one acre
of area with a point source discharge to surface waters of the commonwealth
will need to obtain a National Pollution Discharge Elimination System
(NPDES) permit for stormwater discharges associated with construction
activities from the Westmoreland County Conservation District prior
to the issuance of a building permit.
B.
Work within natural drainageways shall be subject to permit by PADEP
under 25 Pa. Code Chapter 105.
C.
Any stormwater management facility that would be located in or adjacent
to surface waters of the commonwealth, including wetlands, shall be
subject to permit by PADEP under 25 Pa. Code Chapter 105.
D.
Any stormwater management facility that would be located within,
or discharge onto, a state highway right-of-way, shall be subject
to approval by the Pennsylvania Department of Transportation (PennDOT).
E.
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam, shall be subject to permit by PADEP under 25
Pa. Code Chapter 105.
A.
Any person conducting any land development activities in the Township involving earth disturbance of 5,000 square feet or greater shall submit and obtain approval from the Township of a project plan, in accordance with § 280-14 or 280-15, as applicable, that meets the requirements of this chapter, prior to commencing any construction activity, except as exempted herein, under § 280-8B.
B.
The project plan shall describe the BMPs for erosion and sediment
control, and the postconstruction BMPs for water quality protection.
D.
The requirements in this section apply to the total land development
project, even if it is to take place in stages.
A.
The project plan shall describe how these water quality protection
requirements will be met. Infiltration BMPs shall be evaluated and
utilized to the maximum extent possible to manage the net change in
stormwater runoff generated so that postconstruction discharges do
not degrade the physical, chemical or biological characteristics of
the receiving waters.
B.
Postconstruction stormwater infiltration of runoff shall replicate
preconstruction infiltration of runoff to the maximum extent possible.
C.
In calculating the volume of runoff that can be infiltrated at a
site, the following methodology shall be used:
(1)
Methodology:
|
Rev = [(S)(Rv)(A)]/12
|
|
Where:
| ||||
|
Rev
|
=
|
Recharge volume (acre-feet)
| ||
|
S
|
=
|
Soil-specific recharge factor (inches); see table below
| ||
|
Rv
|
=
|
Volumetric runoff coefficient [0.05 + 0.009(I)], where I is
the impervious surface percentage (impervious area divided by total
project area) x 100%
| ||
|
A
|
=
|
Site area contributing to the recharge facility (acres)
| ||
|
Hydrologic Soil Group (HSG)
|
Soil Specific Recharge Factor(s)
(inches)
| |
|---|---|---|
|
A
|
0.38
| |
|
B
|
0.25
| |
|
C
|
0.13
| |
|
D
|
0.06
|
|
If more than one hydrologic soil group is present at a site,
a composite recharge volume shall be computed based upon the proportion
of total site area within each hydrologic soil group.
|
(2)
In selecting the appropriate infiltration BMPs, the applicant shall
consider the following:
(3)
A detailed soils evaluation of the project site shall be performed
to determine the suitability of infiltration BMPs. The evaluation
shall be performed by a qualified professional, and at a minimum,
address soil permeability, depth to bedrock, susceptibility to sinkhole
formation, and subgrade stability. The general process for designing
the infiltration BMP shall be:
(a)
Analyze hydrologic soil groups (HSG) as well as natural and
man-made features within the watershed to determine general areas
of suitability for infiltration BMPs.
(b)
Provide field testing data to determine appropriate percolation
rate and/or hydraulic connectivity.
(c)
Design infiltration BMPs for required stormwater volume based
on field-determined capacity at the level of the proposed infiltration
surface.
(4)
Soil characteristics, as subject to the specific considerations in Subsection C(7) below:
(a)
Infiltration BMPs are particularly appropriate in hydrologic
soil groups A and B, as described in the Natural Resources Conservation
Service Manual TR-55.
(b)
Low-erodibility factors ("K" factors) are preferred for the
construction of basins.
(c)
There must be a minimum depth of 48 inches between the bottom
of any facility and the seasonal high water table and/or bedrock (limiting
zones), except for infiltration BMPs receiving only roof runoff which
shall be placed in soils having a minimum depth of 24 inches between
the bottom of the facility and the limiting zone.
(d)
There must be an infiltration and/or percolation rate sufficient
to accept the additional stormwater load, and to drain completely
as determined by field tests.
(e)
Infiltration BMPs shall be located a minimum of 10 feet away
from the foundation wall of any building, on the downgradient side.
(f)
The infiltration system shall have positive overflow controls
to prevent storage within one foot of the finished surface or grade.
(g)
Infiltration rates shall not be used in computing the storage
volume of the infiltration system.
(h)
Surface inflows shall be designed to prevent direct discharge
of sediment into the infiltration system.
(5)
The recharge volume provided at the site shall be directed to the
most permeable hydrologic soil group (HSG) available, except where
other considerations apply, such as in limestone geology.
(6)
Any infiltration BMP shall be capable of completely infiltrating
the impounded water within 48 hours.
(7)
Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible areas such as:
(8)
During the period of land disturbance, runoff shall be controlled
prior to entering any proposed infiltration area, and areas proposed
for infiltration BMPs shall be protected from sedimentation and compaction
during the construction phase, so as to maintain their maximum infiltration
capacity.
(9)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
received final stabilization.
A.
Applicants may select a combination of runoff control techniques
which are most suitable to control stormwater runoff from the site.
All controls shall be subject to approval of the Westmoreland County
Conservation District. The Conservation District may request specific
information on design and/or operating features of the proposed stormwater
controls in order to determine their suitability and adequacy in terms
of the standards of this section.
B.
General standards. The following general standards shall be applied
to all development within Upper Burrell Township to promote flow attenuation,
erosion and sediment control and flood control.
(1)
Roof drains are not encouraged to be connected to streets or storm
sewers.
(2)
Runoff shall not be discharged directly onto adjacent property without
the written consent of the adjacent landowner in the form of a recorded
drainage easement.
(3)
Postconstruction rates of runoff from any regulated activity shall
not exceed the peak rates of preconstruction runoff for the following
four different storm frequencies: two-, ten-, twenty-five-, and one-hundred-year.
C.
The applicant shall consider the effect of the proposed stormwater
management techniques on any special soil conditions or geological
hazards which may exist on the development site. In the event that
such conditions are identified on the site, the Westmoreland County
Conservation District may require in-depth studies by a competent
geotechnical engineer.
D.
The stormwater management BMPs shall be selected according to the
following order of preference:
(1)
Site planning for locating proposed buildings, impervious areas and
grading which minimizes disruption of the natural site characteristics.
(2)
Minimization of impervious areas and earthwork.
(3)
Implementation of nonstructural measures.
(4)
Infiltration of runoff on-site.
(5)
Flow attenuation by use of open vegetated swales and natural depressions.
(6)
Stormwater detention/retention structures.
E.
All BMPs shall be designed in accordance with the requirements in
the Pennsylvania Stormwater Best Management Practices Manual, Document
Number: 363-0300-002, effective date: December 30, 2006, or latest
revision. A copy is available at the Township or from PADEP.
F.
Any BMP which is a dam, culvert, stream enclosure or outfall as defined
in 25 Pa. Code Chapter 105 shall be designed according to the requirements
in those regulations.
G.
Any stormwater BMP which does not constitute a dam under 25 Pa. Code
Chapter 105, and is designed to store runoff and requiring a berm
or earthen embankment (i.e., detention basin), shall be designed to
satisfy the following:
(1)
All basins shall be designed to provide for a twenty-four-hour extended detention of the difference between the postconstruction storm and the preconstruction storm (i.e., the postconstruction stormwater runoff will be released over a minimum 24 hours at the ten-year preconstruction rate). Also the basin shall be designed to assure that all other postconstruction peak discharge rates do not exceed the preconstruction discharge rates for the associated required design storms, as specified in § 280-12 of this chapter.
(2)
Berms and earthen embankments shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year post-development
conditions.
(3)
Berms and earthen embankments shall be designed to provide a minimum
1.0 foot of freeboard above the maximum pool elevation computed when
the facility functions for the one-hundred-year postconstruction inflow.
(4)
The slope of all basins shall not exceed three feet horizontal to
one foot vertical, and may be further reduced if the soil has unstable
characteristics, upon recommendation of the Westmoreland County Conservation
District.
(5)
The minimum top of berm width shall be eight feet.
(6)
Outlet pipes shall have a minimum diameter of 15 inches.
(7)
Properly spaced antiseep collars shall be installed on all basin
outlet pipes. Design calculations shall be provided in the project
plan.
(8)
The bottom of all basins shall slope, at 1% minimum, towards an outlet
control structure, and/or an underdrain system shall be installed
to ensure that the basin totally dewaters within 48 hours of the cessation
of the storm.
(9)
A fence, with a minimum height of six feet, must be installed around
all basins. A ten-foot-wide gate with lock and keys must be provided
to allow access for future maintenance.
H.
All stormwater conveyance facilities shall be designed to satisfy
the following:
(1)
Water obstructions shall convey runoff from the twenty-five-year
postconstruction design storm, at a minimum, and shall have a minimum
of 1.0 foot of freeboard measured below the lowest point along the
top of the roadway, without damage to the drainage structure or the
roadway.
(2)
Roadway crossings located within designated floodplain areas must
be able to convey runoff from the one-hundred-year postconstruction
design storm with a minimum of 1.0 foot of freeboard measured below
the lowest point along the top of the roadway, without damage to the
drainage structure or the roadway.
(3)
Drainage conveyances must be able to convey, without damage to the
drainage structure or roadway, runoff from the twenty-five-year postconstruction
design storm without surcharging inlets, where appropriate.
(4)
Adequate erosion and sediment control protection shall be provided
along all open channels, and at all points of discharge.
I.
The design of all stormwater management facilities and BMPs shall
incorporate sound engineering principals and practices. The Township
shall reserve the right to disapprove any design that would result
in the occurrence or continuation of an adverse hydrologic or hydraulic
condition within the watershed.
A.
Runoff calculations for the preconstruction and postconstruction
comparison shall consider the following four different storm frequencies:
two-, ten-, twenty-five-, and one-hundred-year.
B.
Stormwater runoff from all preconstruction and postconstruction sites
shall be calculated using either the Rational Method or a Soil Cover
Complex methodology.
C.
Any stormwater runoff calculations involving areas greater than 200
acres, including on- and off-site areas, shall use a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
method. Table III-1 (below) summarizes acceptable computation methods.
All methods will be selected by the design professional based on the
individual limitations and suitability of each method for a particular
site, and approved by the Westmoreland County Conservation District.
These assumptions shall be used in runoff calculations:
(1)
Average antecedent moisture conditions.
(2)
Type II distribution storm.
(3)
Meadow in good condition shall be used in predevelopment runoff calculations
for all areas of existing cultivation.
(4)
All undeveloped areas other than cultivation shall use the land cover
condition which existed during the past 10 consecutive years.
(5)
All areas to be disturbed during construction and subsequently returned
to open space will be assumed to be reduced one hydrologic group category
level for postdevelopment.
(6)
If the initial condition of the site is undeveloped land, the land
use shall be considered as "meadow" unless the natural land cover
is proven to generate lower runoff curve numbers or Rational "C" value,
such as forested lands.
(7)
Twenty percent of existing impervious area, when present, shall be
considered meadow in good condition in predevelopment runoff calculations
for redevelopment.
D.
All calculations consistent with this chapter using the Soil Cover
Complex method shall be based on the following twenty-four-hour storm
events:
|
Storm Event
|
Rainfall
(inches)
(source: NOAA)
| |
|---|---|---|
|
2-year
|
2.45
| |
|
10-year
|
3.40
| |
|
25-year
|
4.00
| |
|
100-year
|
5.05
|
E.
All calculations using the Rational Method shall use rainfall intensities
in inches per hour consistent with appropriate times of concentration
for overland flow and return periods from the Design Storm Curves
from the PennDOT Region 2, Pennsylvania Rainfall Intensity-Duration-Frequency
Chart shown in the Pennsylvania Department of Transportation Design
Manual, Part 2, January 1990, or latest revision thereof. These charts
are attached hereto as an appendix and are made a part hereof.
F.
If a hydrologic computer model such as TR-20, PSRM or HEC-HMS is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours.
G.
Times of concentration for overland flow shall be calculated using
the methodology presented in Chapter 3 of Urban Hydrology for Small
Watersheds, NRCS, TR-55 (as amended or replaced from time to time
by NRCS). Times of concentration for channel and pipe flow shall be
computed using Manning's equation.
H.
Runoff Curve Numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex method shall be obtained from
Chapter 2 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as
amended or replaced from time to time by NRCS).
I.
Runoff coefficient (C) for both existing and proposed conditions
for use in the Rational Method shall be obtained from any generally
accepted hydrologic reference, as approved by the Westmoreland County
Conservation District.
J.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be obtained from any generally accepted
hydrologic/hydraulic reference, as approved by the Westmoreland County
Conservation District.
K.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method, as approved by the
Westmoreland County Conservation District.
L.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrographs through these facilities using the Storage-Indication
Method. For drainage areas greater than 20 acres in size, the design
storm hydrographs shall be computed using a calculation method that
produces a full hydrograph. The Township may approve the use of any
generally accepted full hydrograph approximation technique, which
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
M.
The Township may require that computed existing runoff rates be reconciled
with field observations and conditions. If the design professional
can substantiate through actual physical calibration that more appropriate
runoff and time-of-concentration values should be utilized at a particular
site, then appropriate variations may be made upon review and recommendations
of the Westmoreland County Conservation District. Calibration shall
require detailed gauge and rainfall data for the particular site in
question.
|
Table III-1
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|---|---|---|---|
|
Method
|
Method Developed By
|
Applicability
| |
|
TR-20 or commercial package based on TR-20
|
USDA-NRCS
|
When use of full model is desirable or necessary
| |
|
TR-55 or commercial package based on TR-55
|
USDA-NRCS
|
Applicable for plans within the models limitations
| |
|
HEC-1
|
U.S. Army Corps of Engineers
|
When full model is desirable or necessary
| |
|
PSRM
|
Penn State University
|
When full model is desirable or necessary
| |
|
Rational Method or commercial package based on rational method
|
Emil Kuiching (1889)
|
For sites less than 200 acres
| |
|
Other methods
|
Various
|
As approved by the Westmoreland County Conservation District
| |
|
Table III-2
Soils Found in Landslide Susceptible Areas
| ||
|---|---|---|
|
Abbreviation
|
Soil Name
| |
|
ErC
|
Ernest silt loam, 8-15% slopes
| |
|
FaD
|
Fairpoint very channery silt loam, 3-8% slopes
| |
|
GcB
|
Gilpin channery silt loam, 3-8% slopes
| |
|
GcC
|
Gilpin channery silt loam, 8-15% slopes
| |
|
GcD
|
Gilpin channery silt loam, 15-25% slopes
| |
|
GcF
|
Gilpin channery silt loam, 25-75% slopes
| |
|
GuB
|
Gilpin Upshur complex, 3-8% slopes
| |
|
GuC
|
Gilpin Upshur complex, 8-15% slopes
| |
|
GuD
|
Gilpin Upshur complex, 15-25% slopes
| |
|
GuF
|
Gilpin Upshur complex, 25-75% slopes
| |
|
GwF
|
Gilpin-Weikert channery loam, 25-75% slopes
| |
|
Ho
|
Holly silt loam
| |
|
MeD
|
Matewan channery loam, 15-25% slopes
| |
|
ShF
|
Shelocta-Gilpin channery silt loam, 25-75% slopes
| |
|
UwD
|
Urban land-Wharton complex, 8-25% slopes
| |
|
VaB
|
Vandergrift silt loam, 3-8% slopes
| |
|
VaC
|
Vandergrift silt loam, 8-15% slopes
| |
|
VaD
|
Vandergrift silt loam, 15-25% slopes
| |
